fix github pull request #418 - towel wetness

Fire damage would dry out a wet towel but never all the way to 0.
Water damage would wet a towel but if it was already wet, its
wetness might decrease.

This uses the pull request's change for increasing the wetness
but changes dry_a_towel so that the original code for decreasing
that will work as is.  Using wet_a_towel() to set wetness to 0
doesn't make much sense, so still won't do so; dry_a_towel() does
and now will.

This also adds missing perm_invent update for towels in inventory
changing wetness.

Fixes #418

doc/fixes37.0

@@ -332,6 +332,10 @@ selling a container to a shop for gold leaves any contents that the shop
 	contents without giving any additional credit; mark out of place
 	contents 'no_charge' so that hero can reclaim them without buying
 add some new demonic and angelic maledictions
+when fire damage dried a wet towel, it would never reduce the wetness to 0
+when water damage wet a towel, the new wetness might randomly become less
+when the wetness of a towel in inventory changed, persistent inventory wasn't
+	updated to show that
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository